Subject: Re: [PATCH 2/3] tun: free skb in early errors

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 1 Dec 2017 16:36:49 +0200

On Fri, Dec 01, 2017 at 03:07:44PM +0800, Jason Wang wrote:
> 
> 
> On 2017年12月01日 13:54, [email-masked] wrote:
> > From: Wei Xu <[email-masked]>
> > 
> > tun_recvmsg() supports accepting skb by msg_control after
> > commit ac77cfd4258f ("tun: support receiving skb through msg_control"),
> > the skb if presented should be freed within the function, otherwise it
> > would be leaked.
> > 
> > Signed-off-by: Wei Xu <[email-masked]>
> > Reported-by: Matthew Rosato <[email-masked]>
> > ---
> >   drivers/net/tun.c | 14 +++++++++++---
> >   1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index 6a7bde9..5563430 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -2067,14 +2067,17 @@ static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t total_len,
> >   {
> >   	struct tun_file *tfile = container_of(sock, struct tun_file, socket);
> >   	struct tun_struct *tun = tun_get(tfile);
> > +	struct sk_buff *skb = m->msg_control;
> >   	int ret;
> > -	if (!tun)
> > -		return -EBADFD;
> > +	if (!tun) {
> > +		ret = -EBADFD;
> > +		goto out_free_skb;
> 
> Unfortunately, you can't to there since tun is NULL.

Right, this should just be kfree_skb(skb); return -EBADFD;

> 
> > +	}
> >   	if (flags & ~(MSG_DONTWAIT|MSG_TRUNC|MSG_ERRQUEUE)) {
> >   		ret = -EINVAL;
> > -		goto out;
> > +		goto out_free_skb;
> >   	}
> >   	if (flags & MSG_ERRQUEUE) {
> >   		ret = sock_recv_errqueue(sock->sk, m, total_len,
> > @@ -2087,6 +2090,11 @@ static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t total_len,
> >   		m->msg_flags |= MSG_TRUNC;
> >   		ret = flags & MSG_TRUNC ? ret : total_len;
> >   	}
> > +	goto out;
> 
> We usually don't use goto in the case of success, and you need deal with the
> case skb != NULL but iov_iter_count(to) == 0 in tun_do_read().
> 
> Thanks

I agree, the way to lay this out is:

	tun_put(tun);
	return ret;

err:
	tun_put(tun);
err_tun:
	if (skb)
		kfree_skb(skb);
	return ret;

> > +
> > +out_free_skb:
> > +	if (skb)
> > +		kfree_skb(skb);
> >   out:
> >   	tun_put(tun);
> >   	return ret;